Bending Characteristic of a Bridge-Type Mechanical Lap Joint of REBCO Tapes

This study focuses on bending characteristic of a bridge-type mechanical lap joint of Rare-Earth Barium Copper Oxide (REBCO) tapes, which has been proposed for segment fabrication of high-temperature superconducting magnet especially for a heliotron-type fusion reactor. Previous studies have developed the joint with a straight geometry, showing sufficiently low joint resistance from a view point of electrical power required to run a cryoplant for removing heat from a superconducting magnet, though conductors connected at the joint section are ideally curved to form a helical-shaped coil. In this study, we experimentally evaluated joint resistance as a function of bending strain for a bridge-type mechanical lap joint of REBCO tapes with indium foils inserted between the joint surfaces. The results showed critical bending strain, where joint resistance starts to increase, was almost the same as irreversible strain of the REBCO tape in the case of a 10-mm-long joint. The critical bending strain varied as a function of both joint resistivity and joint length; lower joint resistivity and longer joint can achieve higher critical bending strain. The fabrication process 'Joining-then-bending' can be also applied to a curved joint for reactor-scaled helical coils, where 25-mm-long joint is planned to be applied, though 'Bending-then-joining' would be needed for the joint of small prototype coils.